This invention relates to a method of and apparatus for processing video data signals, to combine a plurality of video data signals into a multiplexed signal.
Video time-division multiplexers (video TDM""s) are known in which a plurality of input video data signals, typically from several video cameras, are combined into a single video data stream. This data stream comprises interleaved time-sliced video data or xe2x80x9cmap shotsxe2x80x9d from each of the cameras or other video input sources. A typical time-slice or snap shot of a video TDM signal is one field of an input video data signal. Each snap shot or field is typically marked so that the multiplexed video data stream can be separated and reconstructed or decoded into its component parts, which are the original separate input video data signals. The use of video TDM""s permits data from a plurality of video sources to be recorded or transmitted on a single medium by time sharing the medium between the different sources, reducing the capital and operating costs of the overall installation.
A disadvantage of known video TDM""s is that the time division multiplexed output, by its definition is non-continuous for any one video input source. This is because, for any one video input source, there will be periods of time (during which the remaining of the plurality of the video input sources each have a field inserted into the multiplexed video data signal) that no field or snap shot from that particular video source is inserted into the multiplexed signal. The problem is exacerbated in the case of non-synchronised video sources.
It is an object of the invention to minimise the amount of data which is lost due to this phenomenon.
According to the invention a method of processing video signals comprises:
(a) selecting one of a plurality of video input signals;
(b) applying the selected video input signal to an analog to digital video decoder;
(c) deriving from the selected video input signal an early synchronisation signal corresponding to the start of a field of the signal;
(d) applying the early synchronisation signal to a control input of the video decoder to enable an output of the video decoder;
(e) reading data from the enabled output corresponding to the first available field of the selected video input signal;
(f) repeating steps (a) to (e) for further selected video input signals; and
(g) combining the data read from the enabled output of the video decoder in each step (e) to form a time division multiplexed video signal.
The steps (a) to (f) may be carried out simultaneously and independently for successively selected input video signals, to allow successive fields of the time division multiplexed video signal to be combined without delays caused by lack of synchronisation between the selected video input signals.
The early synchronisation signal is preferably derived from the selected video input signal by independently extracting vertical synchronisation information from the signal while the signal is undergoing analog to digital conversion.
Preferably, the time division multiplexed video signal data read from the output of the video decoder is encoded by a digital to analog video encoder for storage or transmission in analog form.
Further according to the invention apparatus for processing video signals comprises:
switch means for selecting any one of a plurality of video input signals;
an analog to digital video decoder connected to an output of the switch means;
a synchronisation circuit associated with the video decoder for generating an early synchronisation signal from a selected video input signal corresponding to the start of a field of the signal and for applying the early synchronisation signal to a control input of the video decoder to enable an output thereof;
storage means connected to the output of the video decoder for storing data therefrom corresponding to the first available field of the selected video input signal; and
encoder means for combining data from the storage means corresponding to successive fields of selected video input signals into a time division multiplexed video signal.
The synchronisation circuit may comprise a video sync separator arranged to extract vertical synchronisation information from the video input signal independently while the signal is undergoing analog to digital conversion.
The synchronisation circuit preferably further includes timer means arranged to generate an early vertical synchronisation signal derived from a composite sync signal output of the video sync separator.
The storage means may comprise at least one random access memory arranged to store data corresponding to a single field of video data.
The invention extends to apparatus for processing a plurality of unsynchronised video signals comprising the above defined apparatus arranged in dual channels and including control means for selecting different video sources for each of the channels, and for combining stored data from the respective channels corresponding to successive fields of the different video sources.